I really does depend on what you're emulating. If you emulate PSX and have 40 PSX ISOs, then it will fill up many DVDs. But most people are only emulating early Nintendo and Sega consoles, in which case, as Mullet said they'll fit on a DVD easily (as long as you don't have several GoodSets, then you'll need a few. ).

To answer your main question, again as Mullet said, DVDs can certainly be used for data rather than just audio. They're essentially very high-capacity CDs.